CN113950538A

CN113950538A - Hot-dip galvanizing treatment method, production method of hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method, and production method of hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method

Info

Publication number: CN113950538A
Application number: CN202080042506.2A
Authority: CN
Inventors: 小西刚嗣
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-06-13
Filing date: 2020-06-03
Publication date: 2022-01-18
Anticipated expiration: 2040-06-03
Also published as: JPWO2020250759A1; CN113950538B; JP7136349B2; WO2020250759A1

Abstract

Providing: a hot dip galvanizing treatment method capable of suppressing the generation of dross defects and promoting alloying when manufacturing an alloyed hot dip galvanized steel sheet. The hot-dip galvanizing method of the present embodiment is a hot-dip galvanizing method for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet. The hot-dip galvanizing treatment method comprises the following steps: a sample collection step (S1), a zeta-phase dross amount determination step (S2), and an operation condition adjustment step (S3). In the sample collection step (S1), a sample is collected from a hot dip galvanizing bath containing Al. In the ζ -phase scum amount determining step (S2), the ζ -phase scum amount in the collected sample is determined. In the operation condition adjustment step (S3), the operation conditions for the hot-dip galvanizing treatment are adjusted based on the calculated ζ -phase dross amount.

Description

Hot-dip galvanizing method, method for manufacturing alloyed hot-dip galvanized steel sheet using same, and method for manufacturing hot-dip galvanized steel sheet using same

Technical Field

The present invention relates to: a hot-dip galvanizing method, a method for manufacturing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.

Background

A hot-dip galvanized steel sheet (hereinafter, also referred to as GI) and an alloyed hot-dip galvanized steel sheet (hereinafter, also referred to as GA) are manufactured by the following manufacturing steps. First, a steel sheet to be subjected to hot dip galvanizing (base steel sheet) is prepared. The base steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. When the base steel sheet is a hot-rolled steel sheet, for example, a hot-rolled steel sheet subjected to pickling is prepared. A hot-rolled steel sheet having a Ni layer formed on the surface thereof by performing Ni pre-plating treatment on the pickled hot-rolled steel sheet as necessary can be prepared. A hot-rolled steel sheet subjected to a treatment other than the above treatment may be prepared. When the base steel sheet is a cold-rolled steel sheet, for example, an annealed cold-rolled steel sheet is prepared. A cold-rolled steel sheet having an Ni layer formed on the surface thereof by performing Ni pre-plating treatment on the annealed cold-rolled steel sheet as necessary may be prepared. A cold-rolled steel sheet subjected to a treatment other than the above treatment may be prepared. The prepared base steel sheet (the hot-rolled steel sheet or the cold-rolled steel sheet) is immersed in a hot-dip galvanizing bath, and subjected to a hot-dip galvanizing treatment to produce a hot-dip galvanized steel sheet. When producing the alloyed hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet is further subjected to heat treatment in an alloying furnace to produce the alloyed hot-dip galvanized steel sheet.

The details of the hot-dip galvanizing treatment in the manufacturing process of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are as follows. A hot-dip galvanizing facility used in a hot-dip galvanizing process includes: the hot dip galvanizing system comprises a molten zinc pot containing a hot dip galvanizing bath, a sink roll arranged in the hot dip galvanizing bath, and a gas wiping device.

In the hot dip galvanizing process, a steel sheet (base steel sheet) is immersed in a hot dip galvanizing bath. Then, the traveling direction of the steel sheet is switched upward by the sink roll disposed in the hot dip galvanizing bath, and the steel sheet is pulled up from the hot dip galvanizing bath. In the steel sheet which has been pulled up and then moved upward, wiping gas is blown from a gas wiping device onto the surface of the steel sheet to scrape off excess molten zinc, thereby adjusting the amount of plating deposited on the surface of the steel sheet. The hot dip galvanizing treatment step is performed by the above method. When manufacturing an alloyed hot-dip galvanized steel sheet, the steel sheet with the amount of the plating layer attached adjusted is further charged into an alloying furnace and subjected to alloying treatment.

In the hot dip galvanizing treatment, Fe is eluted from the steel sheet immersed in the hot dip galvanizing bath into the hot dip galvanizing bath. Fe eluted from the steel sheet into the hot dip galvanizing bath reacts with Al and Zn present in the hot dip galvanizing bath to generate intermetallic compounds called dross. Top and bottom dross are present in the dross. The top dross is dross floating on the liquid surface of the hot dip galvanizing bath, which has a lower specific gravity than the intermetallic compounds of the hot dip galvanizing bath. The bottom dross is dross deposited on the bottom of a molten zinc pot with a specific gravity greater than that of the intermetallic compounds of the hot dip galvanizing bath. Among these dross, in particular, in the hot dip galvanizing treatment, the bottom dross is rolled up from the bottom of the deposited molten zinc pot and floats in the hot dip galvanizing bath along with the accompanying flow generated as the steel sheet in the hot dip galvanizing bath travels. Such floating bottom dross sometimes adheres to the surface of the steel sheet in the hot dip galvanizing process. The bottom dross adhering to the surface of the steel sheet may have a point-like defect on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. In the present specification, such surface defects caused by the bottom dross are referred to as "dross defects". The dross defect deteriorates the appearance of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet, or deteriorates the corrosion resistance. Therefore, it is preferable to suppress generation of scum defects.

Techniques for suppressing generation of scum defects are proposed in japanese patent application laid-open nos. 11-350096 (patent document 1) and 11-350097 (patent document 2).

In patent document 1, in the method for producing an alloyed hot-dip galvanized steel sheet, when the molten zinc bath temperature is T (° c) and the boundary Al concentration defined by Cz ═ 0.0015 × T +0.76 is Cz (wt%), the molten zinc bath temperature T is maintained in the range of 435 to 500 ℃, and the Al concentration in the bath is maintained in the range of Cz ± 0.01 wt%.

In patent document 2, in the method for producing an alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is maintained within a range of 0.15 ± 0.01 wt%. Specifically, patent document 2 describes the following. When the Al concentration in the bath is 0.15 wt% or more, the dross becomes Fe-Al phase, and when the Al concentration in the bath is 0.15 wt% or less, the dross becomes Delta₁Phase (delta)₁Phase). If the dross is in the Fe-Al phase and delta₁When the phase change is repeated, the dross is reduced to a fine size. Therefore, patent document 2 describes that dross can be made fine by maintaining the Al concentration in the bath within a range of 0.15 ± 0.01 wt%, and as a result, generation of dross defects can be suppressed.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 11-350096

Patent document 2: japanese laid-open patent publication No. 11-350097

Non-patent document

Non-patent document 1: practical Applications of Phase diagnostics in Continuous galvanization, Nai-Yong Tang, Journal of Phase Equilibria and Diffusion Vol.27No.5, 2006

Disclosure of Invention

Problems to be solved by the invention

Studies to date have reported the presence of Fe in dross that may be generated during hot dip galvanizing₂Al₅Zn_x(so-called top dross), Delta₁Phase, Gamma₁Phase (gamma)₁Phase), Zeta phase (Zeta phase) 4 kinds. For example, patent document 2 proposes that the concentration of Al in the bath be Fe₂Al₅And delta phase₁Operating in a manner near the phase boundary such that δ is a main factor of dross defect₁The phases are refined.

However, even when the method proposed in patent document 1 or patent document 2 is used, dross defects may still occur on the surface of the galvannealed steel sheet or the hot-dip galvanized steel sheet.

Further, in recent years, there has been an increasing demand for hot dip galvannealing of steel containing a large amount of alloying elements, such as high tensile steel. It is known that high tensile steel containing a large amount of alloying elements is difficult to alloy in an alloying treatment after a hot dip galvanizing treatment. Therefore, a steel sheet made of high-tensile steel is sometimes referred to as a difficult-to-alloy material. A hot dip galvanizing method is required which facilitates alloying even for materials difficult to alloy. In addition, a hot dip galvanizing method that can promote alloying treatment is preferable in the case of manufacturing an alloyed hot dip galvanized steel sheet, even if the material is not a difficult-to-alloy material.

An object of the present disclosure is to provide: a hot-dip galvanizing method capable of suppressing the occurrence of dross defects and promoting alloying, a method for producing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for producing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.

Means for solving the problems

The disclosed hot-dip galvanizing treatment method is used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, and comprises the following steps:

a sample collection step of collecting a sample from an Al-containing hot-dip galvanizing bath;

a zeta-phase dross amount determination step of determining the zeta-phase dross amount in the hot-dip galvanizing bath using the collected sample; and the combination of (a) and (b),

and an operation condition adjustment step of adjusting operation conditions for the hot-dip galvanizing treatment based on the calculated zeta-phase dross amount.

The disclosed method for producing a galvannealed steel sheet comprises the steps of:

a hot-dip galvanizing step of forming a hot-dip galvanized layer on the surface of the steel sheet by applying the hot-dip galvanizing method to the steel sheet; and the combination of (a) and (b),

and an alloying step of alloying the steel sheet having the hot-dip galvanized layer formed on the surface thereof to produce an alloyed hot-dip galvanized steel sheet.

The disclosed method for producing a hot-dip galvanized steel sheet comprises the following hot-dip galvanizing treatment steps:

the steel sheet is subjected to the hot-dip galvanizing treatment to form a hot-dip galvanized layer on the surface of the steel sheet.

ADVANTAGEOUS EFFECTS OF INVENTION

The disclosed hot-dip galvanizing method can suppress the generation of dross defects and promote alloying even when a steel sheet of high-tensile steel is subjected to hot-dip galvanizing and alloying treatments. Further, the method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure can manufacture an alloyed hot-dip galvanized steel sheet in which the generation of dross defects is suppressed, and can promote alloying even when a steel sheet of high-tensile steel is subjected to a hot-dip galvanizing treatment and an alloying treatment. The disclosed method for producing a hot-dip galvanized steel sheet can produce a hot-dip galvanized steel sheet in which the occurrence of dross defects is suppressed.

Drawings

Fig. 1 is a functional block diagram showing the overall configuration of a hot-dip galvanizing line facility for manufacturing an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet.

Fig. 2 is a side view of the hot-dip galvanizing apparatus of fig. 1.

Fig. 3 is a side view of a hot-dip galvanizing apparatus constructed differently from fig. 2.

Fig. 4 is a side view of a hot-dip galvanizing apparatus having a different configuration from fig. 2 and 3.

Fig. 5 is a functional block diagram showing the overall configuration of the hot-dip galvanizing line equipment different from that of fig. 1.

Fig. 6 is a flowchart showing the steps of the hot-dip galvanizing treatment method according to the present embodiment.

Fig. 7 is a diagram showing an example of a photographic image of a part of the observation field of view of a sample collected in the sample collection step of the hot-dip galvanizing treatment method according to the present embodiment.

Detailed Description

[ factors for generating dross defects ]

As described above, in the conventional studies, the following types of dross generated in the hot dip galvanizing process have been reported.

(1)Fe₂Al₅Zn_x

(2)δ₁Phase scum

(3)Γ₁Phase scum

(4) Zeta phase dross

Fe₂Al₅Zn_xKnown as top dross. The specific gravity of the top slag is smaller than that of the hot dip galvanizing bath. Therefore, the top dross easily floats on the liquid surface of the hot dip galvanizing bath. Fe₂Al₅Zn_xHas an orthorhombic crystal structure and a chemical composition consisting of, in mass%, 45% of Al, 38% of Fe and 17% of Zn. The top dross floats on the surface of the hot dip galvanizing bath and can be continuously recovered. Therefore, it is known that the top dross does not easily become a factor of the dross defect.

δ₁Phase dross, gamma₁Phase dross and zeta-phase dross are referred to as bottom dross. The specific gravity of the bottom slag is larger than that of the hot dip galvanizing bath. Therefore, the bottom dross is easily deposited on the bottom of the molten zinc pot storing the hot-dip galvanizing bath.

δ₁The crystal structure of the phase dross is hexagonal, and the chemical composition thereof is 1% by mass or less of Al, 9% by mass or more of Fe, and 90% by mass or more of Zn. Gamma-shaped₁The crystal structure of the phase dross is face centered cubic, and the chemical composition thereof is about 20% by mass of Fe and about 80% by mass of Zn. The zeta-phase dross has a monoclinic crystal structure and has a chemical composition of, by mass%, 1% or less of Al, about 6% of Fe, and about 94% of Zn.

In previous studies, it was reported that the major factor of many dross defects was δ₁An example of phase scum. In patent documents 1 and 2, δ is also considered to be₁Phase scum is one of the factors of scum defect. Therefore, the present inventors thought δ at first time₁Phase scumIs a main factor of scum defect and is researched and researched. However, δ is suppressed even in the hot dip galvanizing treatment₁When phase dross is generated, dross defects may still occur on the surfaces of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet.

Therefore, the present inventors considered that the generation factor of the scum defect is not δ₁Phase scum and possibly other scum. Therefore, the present inventors re-analyzed the chemical composition and crystal structure of the dross defect portion using the alloyed hot-dip galvanized steel sheet generating the dross defect. The inventors further conducted a new analysis of the kind of dross generated in the hot dip galvanizing bath. As a result, the present inventors have obtained the following findings on the dross defect, which are different from the conventional findings.

First, the chemical composition of the dross defect portion on the surface of the alloyed hot-dip galvanized steel sheet was analyzed by using EPMA (Electron Probe Micro Analyzer: Electron Beam Micro Analyzer). Further, the crystal structure of the dross defect portion was analyzed by using a TEM (Transmission Electron Microscope). As a result, the chemical composition of the dross defect portion was 2% by mass of Al, 8% by mass of Fe, and 90% by mass of Zn, and the crystal structure was face-centered cubic.

Delta, which has been considered as a major factor of dross defects₁The chemical composition of the phase dross (1% or less of Al, 9% or more of Fe, and 90% or more of Zn by mass%) is similar to that of the above-described dross defect portion. However, delta₁The crystal structure of the phase dross is hexagonal, not face-centered cubic as specified in the dross defect fraction. Therefore, the present inventors considered that δ, which has been conventionally considered as a main factor of the dross defect, was₁Phase scum is not actually a major factor in scum defects.

Therefore, the present inventors have specified dross which causes the dross defect. In the dross of the above (1) to (4), Fe₂Al₅Zn_xThe chemical composition of the (top dross) is substantially different from the chemical composition of the dross defect fraction. Gamma-shaped₁The crystal structure of the phase scum is face-centered cubic crystal which is the same as the scum defect partHowever, the chemical composition thereof (20% of Fe and 80% of Zn by mass%) is greatly different from that of the dross defect portion. The chemical composition of the ζ -phase dross (Al of 1% or less, Fe of about 6%, and Zn of about 94% in mass%) is different from that of the dross defect portion, and further, the crystal structure (monoclinic crystal) is also different from that of the dross defect portion (face-centered cubic crystal).

Based on the above results of investigation, the present inventors considered that the scum defect is not caused by the scum according to the above (1) to (4). Further, the present inventors considered that the scum defect may be caused by other types of scum than the above-described scum (1) to (4).

Therefore, the present inventors further performed an analysis of dross in the hot dip galvanizing bath. The EPMA and TEM described above were used for the analysis of scum. As a result, the present inventors newly found that Gamma is present as dross generated in the hot dip galvanizing bath₂Phase (gamma)₂Phase) dross.

Γ₂The chemical composition of the phase dross was 2% by mass of Al, 8% by mass of Fe, and 90% by mass of Zn, and the chemical composition of the dross defect portion after the above analysis was the same as that of the above-described dross defect portion. Further, Γ₂The crystal structure of the phase scum is face-centered cubic crystal, which is consistent with the crystal structure of the scum defect part. Accordingly, the inventors believe that Γ₂Phase scum can be a major factor in scum defects. Also, Γ₂The specific gravity of the phase dross is greater than that of the hot dip galvanizing bath, and therefore, Γ₂The phase dross belongs to bottom slag which can be deposited on the bottom of a molten zinc pot.

As mentioned above, Fe₂Al₅Zn_xThe specific gravity of the (top slag) is smaller than that of the hot dip galvanizing bath. Fe₂Al₅Zn_xThe (top dross) floats on the surface of the hot dip galvanizing bath and can be continuously recovered. Thus, Fe₂Al₅Zn_x(top dross) is not likely to cause dross defects.

The inventors further investigated Γ₂The phase dross and the other dross (2) to (4). As a result, it was judged that the dross defect was caused by hard dross, and the dross defect was not easily formed by soft dross.

The results of further studies by the present inventors judged: dross and gamma of the above (2) to (4)₂Gamma-ray in phase scum₂The phase scum is hard scum. Further, it is determined that₁Phase dross to zeta-phase dross ratio gamma₂The phase scum is softer and therefore less likely to be a scum defect. It was also determined that the ζ -phase dross is the softest dross among the dross in the above (2) to (4), and the ζ -phase dross is the least likely to cause dross defects.

Based on the above research results, the present inventors concluded that: the main factor of dross defects generated on the surfaces of the galvannealed steel sheet and the galvannealed steel sheet subjected to the hot-dip galvanizing treatment is Γ₂Phase skimming instead of delta₁And (4) phase scum. Further, the present inventors obtained the following findings: the dross classified as bottom dross is Γ₂Phase dross, delta₁Phase dross, zeta-phase dross and gamma prime₁Any of the phase skimmings, but substantially no Γ is present in the hot dip galvanizing bath₁And (4) phase scum.

The present inventors have further obtained the following findings. The zeta phase dross and the other phase dross phase change with each other. I.e. Γ₂The phase scum and the scum of the zeta phase change phase with each other. That is, Γ occurs depending on the conditions of the hot dip galvanizing process₂Dross with phase dross phase changed to zeta-phase or zeta-phase dross phase changed to gamma₂And (4) phase scum. Therefore, if the ratio of ζ -phase dross in the bottom dross in the hot-dip galvanizing bath is increased, Γ in the hot-dip galvanizing bath becomes larger₂The amount of phase scum is relatively reduced.

Based on the above findings, the present inventors found that: if the operating conditions of the hot dip galvanizing treatment are adjusted to intentionally increase the highest softening zeta-phase dross which has not been paid attention to conventionally and is not easily defective, the hard gamma which is easily defective in dross in the hot dip galvanizing bath can be reduced₂The amount of phase scum is such that scum defects can be suppressed. In the hot dip galvanizing treatment method, it is considered that the above-described operation can be performed by controlling the amount of ζ -phase dross in the hot dip galvanizing bath.

[ concerning alloying treatment ]

The present inventors further studied the case where the alloying treatment is performed after the hot dip galvanizing treatment. In the alloying treatment, Fe contained in the steel sheet diffuses into a hot-dip galvanized layer formed on the surface of the steel sheet to form an Fe — Zn alloy. It is known that the alloying treatment is easily affected by the Al concentration in the hot dip galvanizing bath. When the concentration of Al in the hot dip galvanizing bath is high, Al is also contained in a large amount in the hot dip galvanized layer. Al in the hot-dip galvanized layer prevents Fe in the steel sheet from forming an Fe-Zn alloy with Zn in the hot-dip galvanized layer. That is, in consideration of the alloying treatment, it is preferable that the Al concentration in the hot dip galvanizing bath is low.

In addition, high tensile steel contains a large amount of alloying elements such as Si, P, and Mn. The alloying elements prevent Fe in the steel sheet from diffusing into the hot-dip galvanized layer. Therefore, when the hot-dip galvanizing treatment and the alloying treatment are performed on the high-tensile steel, the Al concentration in the hot-dip galvanizing bath is preferably particularly low.

On the other hand, if the Al concentration in the hot dip galvanizing bath is low, Fe eluted from the steel sheet into the hot dip galvanizing bath is likely to react with Zn in the hot dip galvanizing bath. Therefore, if the Al concentration in the hot dip galvanizing bath is low, the amount of bottom dross increases. In the past, it was thought that δ contained in the bottom slag₁The phase scum becomes a cause of scum defects. Therefore, it is considered that if the Al concentration in the hot dip galvanizing bath is reduced, dross defects are likely to occur.

However, the present inventors have studied as a result that it is known that if the operating conditions of the hot dip galvanizing treatment are adjusted in such a manner that the ζ -phase dross is increased, the dross defect can be suppressed even in the case where the Al concentration in the hot dip galvanizing bath is reduced. As described above, the ζ -phase dross is one of the bottom dross. However, since the ζ -phase dross is soft, it is not likely to cause a dross defect. If the Al concentration in the hot dip galvanizing bath can be reduced, the formation of Fe-Zn alloy can be promoted in the alloying treatment. In this case, even high tensile steel is easily alloyed. Namely, the present inventors found that: by adjusting the operating conditions of the hot-dip galvanizing process in such a manner as to increase the ζ -phase dross, dross defects can be suppressed and alloying can be promoted.

As described above, the hot dip galvanizing treatment method according to the present embodiment is based on the findings different from the conventional technical idea, and specifically, as described below.

[1] The hot-dip galvanizing method of (1) is used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet,

the hot-dip galvanizing treatment method comprises the following steps:

a zeta-phase dross amount determining step of determining a zeta-phase dross amount in the hot-dip galvanizing bath using the collected sample; and the combination of (a) and (b),

Here, "adjusting the operating conditions for the hot-dip galvanizing treatment" means adjusting the operating conditions for the hot-dip galvanizing treatment, which can adjust the amount of ζ -phase dross in the hot-dip galvanizing bath. The term "adjustment of the operating conditions of the hot-dip galvanizing process" refers to not only the act of changing the operating conditions of the hot-dip galvanizing process but also the act of maintaining the operating conditions as they are.

According to the hot-dip galvanizing treatment method having the above configuration, the operating conditions of the hot-dip galvanizing treatment method are adjusted so as to increase the amount of zeta-phase dross based on the amount of zeta-phase dross in the hot-dip galvanizing bath obtained using the sample. As described above, in the hot dip galvanizing bath, the amount of zeta-phase dross and Γ phase₂The amount of phase scum has a negative correlation. Specifically, it means that if the amount of zeta-phase dross in the hot-dip galvanizing bath is large, the gamma-phase dross in the hot-dip galvanizing bath is large₂The amount of phase scum is relatively small. Therefore, the zeta-phase dross amount in the hot dip galvanizing bath can be determined, and the zeta-phase dross can be increased by adjusting the operation conditions based on the zeta-phase dross amount determined, thereby reducing the Γ in the hot dip galvanizing bath₂Amount of phase scum. As a result, generation of scum defects can be suppressed. In addition, Γ is reduced by increasing zeta phase dross₂The amount of phase dross can be reduced, and therefore dross defects can be suppressed even if the Al concentration in the hot dip galvanizing bath is reduced. Alloying can be promoted if the Al concentration in the hot dip galvanizing bath can be reduced.

The hot-dip galvanizing treatment method according to the present embodiment can be suitably applied to high-tension steel. The hot dip galvanizing treatment method of the present embodiment can promote alloying of steel other than high tensile steel. Therefore, the hot-dip galvanizing treatment method according to the present embodiment can be suitably applied to steel other than high-tensile steel. Herein, high tensile steel means steel having a tensile strength of 340MPa or more. In the present specification, steel other than high tensile steel means steel having a tensile strength of less than 340 MPa.

[2] The hot dip galvanizing method of (1),

in the zeta-phase dross amount determining step,

the number of ζ -phase dross per a predetermined area was determined as the ζ -phase dross amount using the collected sample.

Here, the predetermined area is not particularly limited. The predetermined area may be, for example, the entire area of an observation field when the zeta-phase scum is observed in a predetermined observation field using a sample, or may be a unit area (cm)²)。

[3] The hot-dip galvanizing method of (1) or (2),

in the above-mentioned operation condition adjustment step,

performing at least one of (A) and (B) based on the obtained amount of zeta-phase scum to increase the amount of zeta-phase scum.

(A) The bath temperature of the hot dip galvanizing bath is adjusted.

(B) The Al concentration of the hot dip galvanizing bath is adjusted.

Both of the above (a) and (B) are effective operation conditions for converting the dross of the other phase into a ζ -phase dross or increasing the generation of ζ -phase dross. Therefore, by performing at least one of (A) and (B), the amount of zeta-phase scum can be increased and Γ can be decreased₂Amount of phase scum.

[4] The hot dip galvanizing method of (1) to (3),

in the above-mentioned operation condition adjustment step,

when the calculated zeta-phase dross amount is lower than a threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the zeta-phase dross amount.

In this case, whether or not the operation condition is changed can be easily determined from the ζ -phase scum amount and the threshold. For example, when the calculated ζ -phase dross amount is lower than the threshold value, the operation condition may be adjusted so that the ζ -phase dross amount increases. More preferably, when the calculated zeta-phase dross amount is less than a threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted so that the zeta-phase dross amount is equal to or more than the threshold value.

[5] The hot-dip galvanizing method of (1) is the hot-dip galvanizing method described in [4],

in the zeta-phase dross amount determining step,

using the collected sample, the number of zeta-phase scum per a predetermined area is determined as the amount of zeta-phase scum,

in the above-mentioned operation condition adjustment step,

the amount of the zeta-phase scum obtained was measured in terms of unit area (1 cm)²) Converted to less than 5.0 pieces/cm²In the case of the above amount, the operation conditions of the hot dip galvanizing treatment are adjusted to increase the amount of the zeta-phase dross.

In this case, by maintaining the zeta-phase scum amount high, Γ is relatively reduced₂And (4) phase scum. As a result, Γ can be further effectively suppressed₂The generation of dross defects caused by phase dross.

[6] The hot-dip galvanizing method of (1) to (5),

in the above-mentioned operation condition adjustment step,

when the Fe concentration in the hot dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot dip galvanizing bath is defined as Y (mass%), the Fe concentration and the Al concentration in the hot dip galvanizing bath are adjusted so as to satisfy the formulas (1) and (2).

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2)

Here, the Fe concentration in the hot dip galvanizing bath is the Fe concentration (Free-Fe concentration) melted in the hot dip galvanizing bath. That is, in the present specification, the "Fe concentration in the hot dip galvanizing bath" refers to the Fe concentration molten in the hot dip galvanizing bath (that is, in the liquid phase) except for the Fe content contained in the dross (top dross and bottom dross). Similarly, the Al concentration in the hot dip galvanizing bath is the Al concentration (Free-Al concentration) melted in the hot dip galvanizing bath. That is, in the present specification, the "Al concentration in the hot dip galvanizing bath" refers to the Al concentration (that is, in the liquid phase) melted in the hot dip galvanizing bath except for the Al content contained in the dross (top dross and bottom dross).

In this case, the amount of ζ -phase dross increases, and as a result, Γ₂The amount of phase scum is relatively reduced. Therefore, Γ can be further effectively suppressed₂The generation of dross defects caused by phase dross.

[7] The hot-dip galvanizing method of (1) is the hot-dip galvanizing method described in [6],

in the above-mentioned operation condition adjustment step,

when the Fe concentration in the hot dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot dip galvanizing bath is defined as Y (mass%), the Fe concentration and the Al concentration in the hot dip galvanizing bath are adjusted so as to satisfy expressions (1) and (3).

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3)

In this case, the amount of ζ -phase dross further increases, and as a result, Γ₂The amount of phase scum is relatively further reduced. Therefore, Γ can be further effectively suppressed₂The generation of dross defects caused by phase dross.

[8] The hot-dip galvanizing method of (1) to (7),

a sink roll for contacting the steel sheet immersed in the hot dip galvanizing bath and vertically switching the traveling direction of the steel sheet is disposed in a molten zinc pot storing the hot dip galvanizing bath,

in the step of collecting the sample, the sample is collected,

the sample is collected from a depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the molten zinc pot.

In this case, samples were taken from the same depth range as the sink roll. Therefore, the correlation between the amount of ζ -phase dross and dross defects can be further improved.

[9] The method for producing the galvannealed steel sheet according to (1) comprises the steps of:

a hot-dip galvanizing treatment step of subjecting a steel sheet to the hot-dip galvanizing treatment method according to any one of [1] to [8] to form a hot-dip galvanized layer on the surface of the steel sheet; and the combination of (a) and (b),

and an alloying step of alloying the steel sheet having the hot-dip galvanized layer formed on the surface thereof to produce the alloyed hot-dip galvanized steel sheet.

The hot-dip galvanizing treatment method of the present embodiment is applied to the method for manufacturing an alloyed hot-dip galvanized steel sheet of the present embodiment. Therefore, an alloyed hot-dip galvanized steel sheet in which dross defects are suppressed can be produced. Further, alloying can be promoted even when the high-tensile steel is subjected to hot-dip galvanizing treatment and alloying treatment.

[10] The method for producing a hot-dip galvanized steel sheet according to (1) includes the following hot-dip galvanizing treatment steps:

a steel sheet is subjected to the hot-dip galvanizing treatment method described in any one of [1] to [8], and a hot-dip galvanized layer is formed on the surface of the steel sheet.

The hot-dip galvanizing treatment method of the present embodiment is applied to the method for manufacturing a hot-dip galvanized steel sheet of the present embodiment. Therefore, a hot-dip galvanized steel sheet with suppressed dross defects can be produced.

Hereinafter, a hot-dip galvanizing treatment method, a method for manufacturing an alloyed hot-dip galvanized steel sheet, and a method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment will be described with reference to the drawings. In the present specification and the drawings, components having substantially the same function are denoted by the same reference numerals, and description thereof will not be repeated.

[ constitution of Equipment on Hot-Dip galvanizing line ]

Fig. 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line facility for manufacturing an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet. Referring to fig. 1, a hot-dip galvanizing line facility 1 includes: an annealing furnace 20, a hot-dip galvanizing facility 10, and a temper rolling mill (finisher) 30.

The annealing furnace 20 includes: not shown in the figure, 1 or more heating zones, and 1 or more cooling zones disposed downstream of the heating zones. In the annealing furnace 20, the steel sheet is supplied to the heating zone of the annealing furnace 20, and the steel sheet is annealed. The annealed steel sheet is cooled in a cooling zone and conveyed to a hot dip galvanizing facility 10. The hot-dip galvanizing facility 10 is disposed downstream of the annealing furnace 20. In the hot-dip galvanizing facility 10, a steel sheet is subjected to a hot-dip galvanizing treatment to produce an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The temper rolling mill 30 is disposed downstream of the hot dip galvanizing facility 10. In the temper rolling mill 30, the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet manufactured in the hot-dip galvanizing facility 10 is subjected to soft rolling as necessary, and the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is adjusted.

[ with regard to a hot-dip galvanizing facility 10]

Fig. 2 is a side view of the hot-dip galvanizing apparatus 10 in fig. 1. Referring to fig. 2, the hot-dip galvanizing facility 10 includes: a molten zinc pot 101, a sink roll 107, a support roll 113, a gas wiping device 109 and an alloying furnace 111.

The interior of the annealing furnace 20 disposed upstream of the hot-dip galvanizing facility 10 is isolated from the atmosphere and is maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the continuously conveyed steel sheet S in the heating zone. Thereby, the surface of the steel sheet S is activated, and the mechanical properties of the steel sheet S are adjusted.

The downstream end of the annealing furnace 20 corresponding to the side of the annealing furnace 20 away from the annealing furnace has a space in which the draw-down rolls 201 are disposed. The downstream end of the lehr 20 is connected to the upstream end of the long nozzle 202. The downstream end portion of the long nozzle 202 is immersed in the hot dip galvanizing bath 103. The interior of the long nozzle 202 is isolated from the atmosphere and maintained in a reducing atmosphere.

The steel sheet S, which has been conveyed by the lower rotary rolls 201 in the downward direction, is continuously immersed in the hot dip galvanizing bath 103 stored in the hot dip galvanizing pot 101 through the long nozzle 202. A sink roll 107 is disposed inside the molten zinc pot 101. The sink roll 107 has a rotation axis parallel to the width direction of the steel sheet S. The axial width of the sink roll 107 is larger than the width of the steel sheet S. The sink roll 107 contacts the steel sheet S and switches the traveling direction of the steel sheet S to the upper side of the hot dip galvanizing facility 10.

The support roll 113 is disposed in the hot dip galvanizing bath 103 and above the sink roll 107. The support roller 113 includes a pair of rollers. The pair of support rollers 113 has a rotation axis parallel to the width direction of the steel sheet S. The support rollers 113 support the steel sheet S conveyed upward by sandwiching the steel sheet S whose traveling direction is switched upward by the sink rollers 107.

The gas wiping device 109 is disposed above the sink roll 107 and the support roll 113 and above the liquid surface of the hot dip galvanizing bath 103. The gas wiping device 109 includes a pair of gas ejecting devices. The pair of gas injection devices have gas injection nozzles facing each other. During the hot dip galvanizing process, the steel sheet S passes between the pair of gas injection nozzles of the gas wiping apparatus 109. At this time, the pair of gas injection nozzles face the surface of the steel sheet S. The gas wiping device 109 blows gas to both surfaces of the steel sheet S pulled up from the hot dip galvanizing bath 103 to scrape off a part of the hot dip galvanizing adhering to both surfaces of the steel sheet S, thereby adjusting the amount of hot dip galvanizing adhering to the surface of the steel sheet S.

The alloying furnace 111 is disposed above the gas wiping device 109. The steel sheet S conveyed upward by the gas wiping apparatus 109 is passed through the interior of the alloying furnace 111, and the steel sheet S is subjected to alloying treatment. The alloying furnace 111 includes a heating zone, a heat retention zone, and a cooling zone in this order from the side where the steel sheet S enters toward the side where the steel sheet S leaves. The heating zone heats the steel sheet S so that the temperature (sheet temperature) thereof becomes substantially uniform. The heat retention area retains the plate temperature of the steel plate S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become an alloyed hot-dip galvanized layer. The cooling zone cools the steel sheet S formed with the alloyed hot-dip galvanized layer. As described above, the alloying furnace 111 performs the alloying treatment using the heating zone, the heat retention zone, and the cooling zone. In the case of manufacturing an alloyed hot-dip galvanized steel sheet, the alloying furnace 111 performs the alloying treatment described above. On the other hand, in the case of manufacturing a hot-dip galvanized steel sheet, the alloying furnace 111 does not perform alloying treatment. In this case, the steel sheet S passes through the non-operating alloying furnace 111. Here, the non-operation means, for example, a state in which the power supply is stopped (non-activated state) in a state in which the alloying furnace 111 is disposed on-line. The steel sheet S passed through the alloying furnace 111 is conveyed to the subsequent step by the upper rotating roll 115.

In the case of manufacturing a hot-dip galvanized steel sheet, as shown in fig. 3, the alloying furnace 111 may be moved offline. In this case, the steel sheet S is conveyed from the upper rotating roll 115 to the subsequent step without passing through the alloying furnace 111.

When the hot-dip galvanizing facility 10 is a facility dedicated to hot-dip galvanized steel sheets, the hot-dip galvanizing facility 10 may not include the alloying furnace 111 as shown in fig. 4.

[ other constitution examples of the apparatus of the Hot-Dip galvanizing line ]

The hot-dip galvanizing line facility 1 is not limited to the configuration shown in fig. 1. For example, when a Ni layer is formed on a steel sheet by performing Ni pre-plating treatment on the steel sheet before the hot-dip galvanizing treatment, a Ni pre-plating facility 40 may be disposed between the annealing furnace 20 and the hot-dip galvanizing facility 10 as shown in fig. 5. The pre-Ni plating apparatus 40 includes an Ni plating tank for storing an Ni plating bath. The Ni plating treatment is performed by an electroplating method. The hot-dip galvanizing line facility 1 shown in fig. 1 and 5 includes an annealing furnace 20 and a temper mill 30. However, the hot-dip galvanizing line facility 1 may not include the annealing furnace 20. The hot-dip galvanizing line facility 1 may not include the temper mill 30. The hot-dip galvanizing line facility 1 may include at least a hot-dip galvanizing facility 10. The annealing furnace 20 and the temper rolling mill 30 may be arranged as required. The hot-dip galvanizing line facility 1 may include a pickling facility for pickling a steel sheet upstream of the hot-dip galvanizing facility 10, or may include a facility other than the annealing furnace 20 and the pickling facility. The hot-dip galvanizing line facility 1 may further include a facility other than the temper mill 30 downstream of the hot-dip galvanizing facility 10.

[ mechanism for generating dross defects ]

In the hot-dip galvanizing treatment step in the manufacturing process of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet using the above-described hot-dip galvanizing line facility 1, the mechanism of dross defect generation is considered as follows.

In the hot-dip galvanizing process, Fe is eluted from the steel sheet S immersed in the hot-dip galvanizing bath 103 into the hot-dip galvanizing bath 103. The leached Fe reacts with Al and/or Zn in hot dip galvanizing bath 103 to generate dross. The top dross in the dross thus generated floats on the surface of the hot dip galvanizing bath 103. On the other hand, the bottom dross in the generated dross settles and is deposited on the bottom of the molten zinc pot 101. When the production of the galvannealed steel sheet or the hot-dip galvanized steel sheet is repeated (that is, as the amount of the steel sheet S passing through the hot-dip galvanizing bath 103 increases), the bottom dross is deposited on the bottom of the molten zinc pot 101.

The bottom dross deposited at the bottom of the molten zinc pot 101 is rolled up into the hot dip galvanizing bath 103 to float in the hot dip galvanizing bath 103 with the accompanying flow of the steel sheet S generated in the vicinity of the lower portion of the sink roll 107. The bottom dross floating in the hot dip galvanizing bath 103 adheres to the surface of the steel sheet S in the vicinity of the sink roll 107. The portion of the bottom dross adhering to the surface of the steel sheet S may become a dross defect.

If a dross defect occurs, uneven portions of the coating layer occur on the surface of the coating layer, and the quality of the appearance of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet deteriorates. Further, local batteries are easily formed in the dross defect portions on the steel sheet surface, and the corrosion resistance of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is lowered.

As mentioned above, the main factor of dross defects is Γ₂Phase scum instead of δ, which has been reported in large numbers in previous studies₁And (4) phase scum. Therefore, if Γ in hot dip galvanizing bath 103₂When the amount of phase dross is large, the possibility of dross defects occurring in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet becomes high.

Further, zeta-phase dross and gamma₂The phase scum changes phase with each other. That is, the zeta-phase dross phase changes to Γ₂Phase dross, Γ₂The phase scum phase changes to zeta-phase scum. Thus, in hot dip coatingIn the zinc bath 103, the amount of zeta-phase dross is equal to gamma-phase₂The amount of phase dross has a negative correlation, and means that if the amount of ζ -phase dross in hot dip galvanizing bath 103 is large, Γ phase in hot dip galvanizing bath 103 is large₂The amount of phase scum is relatively small. Further, the ζ -phase dross is softest as compared with the dross of the other phase, and is less likely to cause a dross defect. Therefore, the zeta-phase dross amount in hot dip galvanizing bath 103 can be reduced by determining the zeta-phase dross amount, adjusting the operating conditions based on the zeta-phase dross amount determined, and increasing the zeta-phase dross amount₂Amount of phase scum. As a result, generation of scum defects can be suppressed.

Therefore, in the hot dip galvanizing treatment method of the present embodiment, the amount of ζ -phase dross in the hot dip galvanizing bath 103 is determined. Then, the operating conditions of the hot dip galvanizing treatment are adjusted based on the amount of ζ -phase dross in the hot dip galvanizing bath 103. It is preferable to adjust the operating conditions of the hot-dip galvanizing treatment in such a manner that the amount of ζ -phase dross is increased based on the amount of ζ -phase dross in the hot-dip galvanizing bath 103. As a result, the amount of zeta-phase dross in hot-dip galvanizing bath 103 can be increased to gamma₂The amount of phase scum is suppressed relatively low. As a result, the generation of dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet can be suppressed. Preferably, the operation conditions of the hot dip galvanizing treatment are adjusted so as to increase the ζ phase dross based on the ζ phase dross amount in the hot dip galvanizing bath 103, and the ζ phase dross amount in the hot dip galvanizing bath 103 is maintained at a specific amount (threshold value) or more.

The hot-dip galvanizing treatment method according to the present embodiment may be applied to a method for manufacturing a galvannealed steel sheet (GA) and may also be applied to a method for manufacturing a hot-dip galvanized steel sheet (GI). The hot-dip galvanizing treatment method according to the present embodiment will be described in detail below.

[ Hot-dip galvanizing treatment method according to the present embodiment ]

[ Hot-dip galvanizing facility for utilization ]

In the hot-dip galvanizing treatment method of the present embodiment, the hot-dip galvanizing line facility 1 is used. The hot-dip galvanizing line facility 1 has a configuration shown in fig. 1 and 5, for example. However, the hot-dip galvanizing line facility 1 used in the hot-dip galvanizing treatment method according to the present embodiment may be the facility shown in fig. 1 and 5 as described above, or a facility having another configuration may be added to the facility shown in fig. 1 and 5. In addition, a known hot-dip galvanizing line facility 1 having a different configuration from that shown in fig. 1 and 5 may be used.

[ Steel sheet for use in Hot-Dip galvanizing treatment ]

The steel type and size (plate thickness, plate width, etc.) of the steel plate (base steel plate) used in the hot dip galvanizing treatment of the present embodiment are not particularly limited. The steel sheet may be a known steel sheet suitable for an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet, depending on various mechanical properties (for example, tensile strength, workability, and the like) required for the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet to be produced. A steel sheet used for an automobile outer panel can be used as a steel sheet (base steel sheet) used in a hot-dip galvanizing process.

As described above, in the hot dip galvanizing treatment of the present embodiment, dross defects can be suppressed even if the Al concentration in the hot dip galvanizing bath 103 is reduced. Therefore, alloying can be promoted by reducing the Al concentration in hot dip galvanizing bath 103. The steel sheet used in the hot dip galvanizing treatment of the present embodiment may be a steel sheet made of high tensile steel containing a large amount of alloying elements such as Si and Mn. The steel sheet used in the hot dip galvanizing treatment of the present embodiment may be a steel sheet made of steel other than high tensile steel.

The steel sheet (base steel sheet) used in the hot dip galvanizing treatment of the present embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet. As the base steel sheet, for example, the following steel sheets are used.

(a) Acid-washed hot-rolled steel plate

(b) Hot-rolled steel sheet having Ni layer formed on surface thereof by pickling and then subjecting the steel sheet to Ni pre-plating

(c) Annealed cold rolled steel sheet

(d) Cold-rolled steel sheet having Ni layer formed on surface thereof by performing Ni preplating treatment after annealing

The above (a) to (d) are examples of steel sheets used in the hot dip galvanizing treatment of the present embodiment. The steel sheet used in the hot dip galvanizing treatment of the present embodiment is not limited to the above (a) to (d). The hot-rolled steel sheet or cold-rolled steel sheet subjected to the treatment other than the above (a) to (d) may be used as a steel sheet for the hot-dip galvanizing treatment.

[ Hot-dip galvanizing bath ]

The main component of hot dip galvanizing bath 103 is Zn. Hot dip galvanizing bath 103 contains Al in addition to Zn. That is, the hot dip galvanizing bath 103 used in the hot dip galvanizing treatment method of the present embodiment is a plating bath containing Al at a specific concentration, and the balance of Zn and impurities. If hot dip galvanizing bath 103 contains Al at a specific concentration, excessive reaction of Fe and Zn in the bath can be suppressed, and uneven alloying reaction between the steel sheet immersed in hot dip galvanizing bath 103 and Zn can be suppressed from proceeding.

The preferable Al concentration (more specifically, Free-Al concentration) in the hot dip galvanizing bath 103 is 0.100 to 0.159% by mass. Here, the Al concentration in hot dip galvanizing bath 103 is the concentration (mass%) of Al dissolved in hot dip galvanizing bath 103, and is the so-called Free-Al concentration. When the Al concentration in the hot dip galvanizing bath is in the range of 0.100 to 0.159% by mass%, the generation of pattern defects other than dross defects can be suppressed, and further, the generation of unalloyed metal can be suppressed in the alloying treatment in the manufacturing process of the alloyed hot dip galvanized steel sheet.

As described above, hot dip galvanizing bath 103 according to the present embodiment is a plating bath containing Zn as a main component and also containing Al. The hot dip galvanizing bath 103 may further contain 0.020 to 0.100 mass% of Fe eluted from facilities and steel sheets in the bath. That is, the concentration (% by mass) of Fe dissolved in hot dip galvanizing bath 103 is, for example, 0.020 to 0.100% by mass. However, the concentration of Fe dissolved in hot dip galvanizing bath 103 is not limited to the above numerical range.

[ Hot Dip galvanizing treatment method ]

The hot dip galvanizing treatment method of the present embodiment uses a hot dip galvanizing bath 103 containing Al. Fig. 6 is a flowchart showing the steps of the hot-dip galvanizing treatment method according to the present embodiment. Referring to fig. 6, the hot dip galvanizing method according to the present embodiment includes: a sample collection step (S1), a zeta-phase dross amount determination step (S2), and an operation condition adjustment step (S3). Hereinafter, each step will be described in detail.

[ sample Collection Process (S1) ]

In the sample collection step (S1), a part of the plating liquid is collected as a sample from the hot dip galvanizing bath 103. In the sample collection step (S1), a sample is collected over time. By "collecting samples over time" is meant that samples are collected every time a particular period of time has elapsed. The specific time (the period after the sample is collected until the next sample is collected) may or may not be constant. For example, samples may be taken every 1 hour. In addition, the next sample may be collected after 1 hour from the collection of the sample, and the next sample may be collected after 30 minutes. The specific time is not particularly limited.

The amount of the sample collected in the autothermal dip galvanizing bath 103 is not particularly limited. In the zeta-phase dross amount determination step (S2) in the subsequent step, the sample collection amount is not particularly limited as long as the amount of zeta-phase dross in hot-dip galvanizing bath 103 can be determined. The sample collection amount is, for example, 100 to 400 g. The collected sample may be brought into contact with a metal having a high thermal conductivity at normal temperature, and the sample may be quenched to normal temperature to be solidified. The normal temperature metal having high thermal conductivity is, for example, copper.

The sample collection position in the hot dip galvanizing bath 103 is not particularly limited. For example, referring to fig. 2 to 4, when the hot dip galvanizing bath 103 is trisected into D1 to D3 in the depth direction D, a sample may be collected in the uppermost region D1, the middle region D2, or the lowermost region D3 in the hot dip galvanizing bath 103. The zeta-phase scum amount of the samples collected in the regions D1 to D3 was different. However, it can be determined to some extent whether the amount of the calculated ζ -phase dross is large or not according to the collecting position. Therefore, the position of collecting the sample is not particularly limited. As shown in fig. 2 to 4, in hot dip galvanizing bath 103, a direction parallel to the plate width direction of steel plate S is defined as a width direction W, a depth direction of hot dip galvanizing bath 103 is defined as a depth direction D, and a direction perpendicular to width direction W and depth direction D is defined as a longitudinal direction L. In this case, it is preferable to collect the sample over time from a specific region divided by a specific width range in the width direction W, a specific depth range in the depth direction D, and a specific length range in the length direction L. In any case, samples are taken over time from the same position (within a specific area) within the hot dip galvanizing bath 103.

It is preferable to try to collect samples from the area near the sink roll 107. Specifically, as shown in fig. 2 to 4, in the hot dip galvanizing bath 103, samples are taken within a specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D. That is, the specific depth range is set to a depth range D107 from the upper end to the lower end of the sink roll 107. Gamma-shaped₂The phase dross is likely to adhere to the surface of the steel sheet S near the sink roll 107. Therefore, the amount of ζ -phase dross in the vicinity of the sink roll 107 is most effective as an index for suppressing the dross defect. Therefore, it is preferable to collect the sample from the depth range D107. In this case, the amount of zeta-phase dross is determined based on a sample collected from a range most likely to adhere to the surface of the steel sheet S, and therefore, the correlation between the amount of zeta-phase dross and dross defects can be further improved. It is also preferable that the samples be collected from the region near the sink roll as much as possible in the width direction W and the length direction L. As described above, samples are collected from the same region in hot dip galvanizing bath 103 over time.

[ Zeta-phase dross amount determining step (S2) ]

In the ζ -phase dross amount determining step (S2), the ζ -phase dross amount in the hot dip galvanizing bath 103 is determined using the collected sample. The method for determining the amount of zeta-phase scum using the sample is not particularly limited, and various methods can be considered.

For example, a zeta-phase dross observation test piece was prepared from the sample collected in the sample collection step (S1). As an example of the test piece for observing ζ -phase scum, a rectangular parallelepiped (platelet shape) having a surface (test surface) capable of securing an observation field of 15mm × 15mm and a thickness of 0.5mm was formed. The dross in the entire field of view was identified by observing the entire field of view (15 mm. times.15 mm) with an optical microscope or Scanning Electron Microscope (SEM) of a predetermined magnification. Scum can be specified according to the contrast in the field of view, and further top scum can be distinguished from bottom scum according to the contrast.

Fig. 7 is an example of a photographic image of a part of the observation field of view of the sample collected in the sample collection step (S1). Referring to fig. 7, in the photographic image, a hot-dip galvanized mother phase 200, top slag 100T, and bottom slag 100B were observed. The brightness of the top slag 100T is lower (darker) than the brightness of the mother phase 200 and the bottom slag 100B. On the other hand, the luminance of the bottom slag 100B is lower (darker) than that of the mother phase 200 and higher (brighter) than that of the top slag 100T. As described above, the top slag and the bottom slag may be distinguished based on the contrast.

For each of the bottom dross specified in the above observation field (15 mm. times.15 mm), composition analysis using EPMA was performed to specify the ζ -phase dross. Further, the crystal structure analysis using TEM was performed on each of the bottom dross, and the ζ -phase dross in the observation field can be specified. Instead of distinguishing the top dross and the bottom dross by contrast, the types of the dross in the field of view (top dross, Γ) may be specified by analyzing the composition of each dross using EPMA and/or by analyzing the crystal structure of each dross using TEM₂Phase dross, delta₁Phase scum and zeta-phase scum).

Based on the specified ζ -phase dross, the ζ -phase dross amount in hot dip galvanizing bath 103 is determined. The amount of ζ -phase dross in the hot dip galvanizing bath 103 can be determined in various indexes. For example, the number of ζ -phase dross per predetermined area may be regarded as the ζ -phase dross amount. Here, the predetermined area is not particularly limited, and may be, for example, the entire area of the observation field of view or a unit area (mm)²). For example, when the observation field of view is 15mm × 15mm, the observation field of view (15mm × 15mm — 225 mm) may be set to²) The number of zeta-phase scum in (number/225 mm)²) As the amount of ζ -phase scum. In this case, the number of ζ -phase scum in the observation field of view was determined by the following method. First, the circle-equivalent diameter (. mu.m) of the specific ζ -phase dross was obtained. The diameter of each zeta-phase dross in the observed field of view when converted into a circle was defined as the circle-equivalent diameter (μm). Using the photographic image of the observation field, the circle-equivalent diameter (μm) of the specific ζ -phase dross was obtained by known image processing. The number of zeta-phase scum having a circle equivalent diameter of 10 μm or more was defined as the number of zeta-phase scum (number/225 mm)²). In this way, the number of ζ -phase dross having a circle-equivalent diameter of 10 μm or more in the observation field of view can be defined as the ζ -phase dross amount. The observation field of view is not limited to the above-described region (15mm × 15mm — 225 mm)²). The upper limit of the circle-equivalent diameter of the ζ -phase dross is not particularly limited. The upper limit of the circle-equivalent diameter of the ζ -phase dross is, for example, 300 μm.

In addition, other indexes may be used as the amount of ζ -phase dross in the hot-dip galvanizing solution. For example, each bottom sediment (each Γ) in the observation field is obtained₂Phase dross, each delta₁Phase scum and each zeta-phase scum) and the area of each zeta-phase scum. Then, the ratio of the total area of the ζ -phase dross with respect to the total area of the bottom dross may be taken as the ζ -phase dross amount. The ratio of the total area of the ζ -phase scum with respect to the observation visual field area can be regarded as the ζ -phase scum amount. In addition, the total area (μm) of the ζ -phase scum in the above-mentioned visual field can be set²) As the amount of ζ -phase scum. Further, the test surface of the sample was subjected to X-ray diffraction measurement to measure each bottom sediment (Γ)₂Phase dross, delta₁Phase scum and zeta phase scum). Then, the sum of the peak intensities (i.e., Γ) relative to the bottom slags may be determined₂Peak intensity of phase scum, delta₁The sum of the peak intensity of the phase dross and the peak intensity of the ζ -phase dross), the ratio of the peak intensity of the ζ -phase dross was taken as the ζ -phase dross amount. In the X-ray diffraction measurement, it is not easy to clearly distinguish Γ₂Phase dross and gamma-ray₁And (4) phase scum. However, as described above, Γ is considered to be₁Phase scum is substantially absent. Therefore, all the peak intensities obtained at diffraction angles 2 θ of 43 to 44 ° are regarded as Γ₂Peak intensity of phase scum. For example, Co dry spheres are used as targets for X-ray diffraction measurement. The amount of ζ -phase scum can be determined by other methods than the above.

By the above method, the amount of ζ -phase dross in the hot dip galvanizing bath 103 is determined using the sample collected in the sample collection step (S1). The ζ -phase dross amount determining step (S2) is preferably performed each time a sample is collected in the sample collecting step (S1). By collecting samples over time and determining the amount of zeta-phase dross each time a sample is collected, the change in the amount of zeta-phase dross in hot-dip galvanizing bath 103 over time can be grasped. Thus, based on the samples collected over time, the amount of zeta-phase scum can be determined over time.

[ Process for adjusting operating conditions (S3) ]

After the zeta-phase dross amount in the hot dip galvanizing bath 103 is determined in the zeta-phase dross amount determination step (S2), the operation condition adjustment step (S3) is performed.

In the operating condition adjusting step (S3), the operating conditions for the hot-dip galvanizing treatment are adjusted based on the amount of ζ -phase dross in the hot-dip galvanizing bath 103. Specifically, when the calculated amount of zeta-phase dross is small, the operating conditions are adjusted (changed) so as to increase the amount of zeta-phase dross in the hot-dip galvanizing bath 103. If the amount of the zeta-phase dross obtained is appropriate, the current operating conditions can be maintained. The method for adjusting the operating conditions is not particularly limited as long as the amount of ζ -phase dross in the hot dip galvanizing bath 103 can be adjusted. Specifically, the method of adjusting the operation conditions is not particularly limited as long as the amount of ζ -phase dross in the hot dip galvanizing bath 103 can be adjusted so as to be increased.

As a method for adjusting the operation conditions, at least one of the following (a) or (B) is preferably performed.

(A) The bath temperature of hot dip galvanizing bath 103 is adjusted.

(B) The Al concentration of hot dip galvanizing bath 103 is adjusted.

With regard to the above (A), if the temperature of hot dip galvanizing bath 103 is raised, Γ₂The possibility that the phase dross phase becomes ζ -phase dross becomes high. Therefore, if the temperature of hot dip galvanizing bath 103 is increased, Γ in hot dip galvanizing bath 103₂Phase scum is reduced and instead, zeta phase scum is increased. As described above, the ζ -phase dross is soft. Therefore, the ζ -phase dross is less likely to form a dross defect. Therefore, when the amount of ζ -phase dross in hot dip galvanizing bath 103 is too small, the bath temperature of hot dip galvanizing bath 103 can be increased. In this case, the hard r₂The phase scum phase changes to a soft zeta-phase scum. As a result, the soft zeta-phase scum increases and the hard gamma-phase increases₂The phase scum is reduced. Thus, suppressing the scum defectAnd (4) generating. It should be noted that increasing the bath temperature increases the energy source unit. Therefore, in the case where the amount of ζ -phase dross is sufficiently large, it is not necessary to excessively raise the bath temperature. As described above, the amount of ζ -phase dross in hot dip galvanizing bath 103 can be adjusted by adjusting the bath temperature of hot dip galvanizing bath 103. Specifically, the bath temperature of hot dip galvanizing bath 103 is increased to increase the zeta-phase dross amount, and as a result, Γ in hot dip galvanizing bath 103 can be reduced₂Amount of phase scum.

As for (B) above, if the Al concentration in hot dip galvanizing bath 103 is reduced, Γ₂The possibility that the phase dross phase becomes ζ -phase dross becomes high. Therefore, when the amount of ζ -phase dross in hot dip galvanizing bath 103 is too small, the amount of ζ -phase dross in hot dip galvanizing bath 103 can be adjusted by adjusting the Al concentration in hot dip galvanizing bath 103. Specifically, by reducing the Al content of hot dip galvanizing bath 103, the ζ -phase dross amount can be increased, and as a result, Γ in hot dip galvanizing bath 103 can be reduced₂And (4) phase scum.

Based on the amount of zeta-phase scum obtained under the above-mentioned operating conditions (a) and (B), only one of the operating conditions may be adjusted, or both of the operating conditions (a) and (B) may be adjusted. For example, when the amount of ζ -phase dross is too small, the bath temperature of hot-dip galvanizing bath 103 may be increased and the Al concentration of hot-dip galvanizing bath 103 may be decreased. When the amount of the ζ -phase dross is appropriate, the current operating conditions of (a) and (B) can be maintained.

The threshold value may be set based on whether or not the ζ -phase dross amount obtained in the ζ -phase dross amount determining step (S2) is appropriate as a determination index. In this case, the operation condition may be adjusted depending on whether the calculated amount of ζ -phase dross is less than the threshold value. Specifically, the operation conditions may be changed or maintained without changing the operation conditions depending on whether the calculated amount of ζ -phase dross is less than the threshold value. For example, when the calculated zeta-phase dross amount is less than the threshold value, it is determined that the zeta-phase dross amount is too small, and the operation conditions are changed so that the zeta-phase dross amount in the hot dip galvanizing bath 103 increases more than it is. Preferably, when the calculated zeta-phase scum amount is less than a threshold value, the operation conditions are changed so that the zeta-phase scum amount is equal to or greater than the threshold value. On the other hand, when the calculated amount of ζ -phase dross is equal to or more than the threshold value, it is determined that the amount of ζ -phase dross in hot dip galvanizing bath 103 is sufficiently large, and the current operating conditions are maintained.

When the number of zeta-phase scum per predetermined area, for example, the number of zeta-phase scum in the above-mentioned observation field is regarded as the zeta-phase scum amount, the number is converted into the number per unit area (1 cm)²) The number of (2) is equivalent to 5.0 pieces/cm²The number of (2) is used as a threshold value. For example, the above observation field of view (15 mm. times.15 mm: 225 mm)²) When the number of ζ -phase dross in (d) was defined as the ζ -phase dross amount, the threshold value was set to 11.25 pieces (5.0 pieces/cm)²×225mm²). In this case, the amount of zeta-phase scum obtained in the zeta-phase scum amount determining step (S2) is greater than the threshold value (11.25 scum/225 mm)²) The number of (1 cm) is expressed in terms of unit area²) Lower than 5.0 pieces/cm in terms of conversion²When the amount of the metal compound (D) is too small, it is judged that the amount of the zeta-phase dross is too small, and the operation conditions are adjusted so that the amount of the zeta-phase dross in the hot dip galvanizing bath 103 increases. Preferably, the amount of zeta-phase scum determined in the zeta-phase scum amount determination step (S2) exceeds the threshold value (11.25 pieces/225 mm)²) When the amount of the zeta-phase scum is less than 5.0 pieces/cm in terms of unit area²The number of (2) is such that the amount of zeta-phase scum becomes the threshold (11.25 pieces/225 mm)²) The above number (that is, the number in terms of unit area is 5.0 pieces/cm²The above number) of the operation conditions. For example, the content of zeta-phase dross determined in the zeta-phase dross content determining step (S2) is less than 5.0 pieces/cm in terms of unit area²In the case of (B), at least one of the above-mentioned operating conditions (a) or (B) is carried out to increase the amount of zeta-phase scum. For example, the bath temperature of hot dip galvanizing bath 103 is increased to increase the amount of ζ -phase dross. In addition, for example, the Al content of hot dip galvanizing bath 103 is reduced to increase the ζ -phase dross amount. The upper limit is not particularly limited, as the number of ζ -phase dross per predetermined area is preferably larger.

Preferably, in the operating condition adjusting step (S3), the Fe concentration and the Al concentration in the hot dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (2) when the Fe concentration in the hot dip galvanizing bath 103 is defined as X (mass%) and the Al concentration in the hot dip galvanizing bath 103 is defined as Y (mass%) based on the ζ -phase dross amount determined in the ζ -phase dross amount determining step (S2).

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2)

Here, the Al concentration is the concentration of Al in the Al in hot dip galvanizing bath 103 excluding the content of Al contained in dross, and is the so-called Free-Al concentration (mass%). Similarly, the Fe concentration means an Fe concentration other than the Fe content contained in the dross in the Fe in hot dip galvanizing bath 103.

Formula (1) represents the range of Al concentration Y (mass%) in hot dip galvanizing bath 103. The Al concentration Y in the hot dip galvanizing bath 103 is related to the top dross and Γ₂Amount of phase scum and zeta-phase scum produced. When the Al concentration Y is 0.139% or less, the transformation of the slag top into Γ is facilitated₂Phase scum and zeta-phase scum. In this case, excessive top dross generation can be suppressed. This can prevent the top dross from being pinched between the sink roll 107 and the steel sheet, thereby preventing surface flaws from being generated. Therefore, in order to suppress the occurrence of surface flaws, the formation of top dross can be suppressed. In order to suppress surface defects, the Al concentration in hot dip galvanizing bath 103 may be kept at 0.140% or less. However, in the actual operation of the hot dip galvanizing process, there is a possibility that fluctuation of ± 0.001% is generated at the maximum in the Al concentration management. Therefore, in formula (1), the upper limit of Al concentration Y in hot dip galvanizing bath 103 is set to 0.139%.

The lower limit of the Al concentration is not particularly limited from the viewpoint of suppressing the occurrence of surface defects. However, it is known that the Al concentration in hot dip galvanizing bath 103 is made constant or higher, whereby the overalloying can be suppressed during the alloying treatment. In the formula (1), the lower limit of the Al concentration (the lower limit of the formula (1)) is set to 0.100%.

The lower limit of the Al concentration Y in hot dip galvanizing bath 103 may be 0.100%, 0.105%, or 0.110%. The upper limit of the Al concentration Y in hot dip galvanizing bath 103 may be 0.139%, 0.135%, 0.130%, and 0.125%.

Formula (2) corresponds to Γ in hot dip galvanizing bath 103₂The phase dross phase changes to the boundary of the zeta-phase dross (phase change line). If the Al concentration Y in hot dip galvanizing bath 103 is higher than the right side of formula (2), the chemical composition of hot dip galvanizing bath 103 becomes Γ₂The phase scum can be in a state in which it is more stably present than the zeta phase scum. In this case, assuming that the Al concentration Y in hot dip galvanizing bath 103 satisfies formula (1), ζ -phase dross is likely to be phase-converted into Γ₂And (4) phase scum. Therefore, Γ is likely to be generated in hot dip galvanizing bath 103₂Phase scum state.

On the other hand, if the Al concentration Y in hot dip galvanizing bath 103 is equal to or less than the right side of formula (2), that is, if the Al concentration Y and the Fe concentration X satisfy formula (2), assuming that the Al concentration Y in hot dip galvanizing bath 103 satisfies formula (1), the chemical composition of hot dip galvanizing bath 103 becomes zeta-phase dross which can be larger than Γ₂The phase scum is also stably present. Thus, Γ in hot dip galvanizing bath 103₂The phase scum is easily phase-changed into zeta-phase scum. Therefore, in hot dip galvanizing bath 103, the steel sheet becomes Γ₂A state in which phase scum is easily reduced.

Therefore, in the above-described hot dip galvanizing treatment, if the Al concentration Y and the Fe concentration X in the hot dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (2), the generation of ζ -phase dross is promoted in the hot dip galvanizing bath 103, and Γ, which has a negative correlation with the ζ -phase dross amount, can be reduced₂Amount of phase scum.

More preferably, in the operating condition adjusting step (S3), the Fe concentration and the Al concentration in the hot dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (3) when the Fe concentration in the hot dip galvanizing bath 103 is defined as X (mass%) and the Al concentration in the hot dip galvanizing bath 103 is defined as Y (mass%) based on the ζ -phase dross amount obtained in the ζ -phase dross amount determining step (S2).

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3)

The formula (3) is a formula specifying a region where the Al concentration is lower than the formula (2). The above formula (2) corresponds to Γ in hot dip galvanizing bath 103₂The phase dross phase changes to the boundary of the zeta-phase dross (phase change line). The formula (3) is a region where the ζ -phase scum can stably exist even in comparison with the specific region in the formula (2). Thus, Γ in hot dip galvanizing bath 103₂The phase scum is further easily phase-changed into zeta-phase scum. Therefore, in hot dip galvanizing bath 103, the steel sheet becomes Γ₂A state in which the phase scum is likely to be further reduced.

The Fe concentration (Free-Fe concentration) in the hot dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot dip galvanizing bath can be determined in the following manner. In the hot dip galvanizing bath 103 of fig. 2, samples are taken from a specific depth range in the depth direction D. More specifically, in hot dip galvanizing bath 103 of fig. 2, a sample is collected from a specific region (hereinafter, referred to as a sample collection region) defined by a specific depth range in depth direction D, a specific width range in width direction W, and a specific length range in length direction L. In the case where samples are sequentially collected over time, the collection positions of the samples are set to the same position (within the same sample collection area). And cooling the collected sample to the normal temperature. The Fe concentration (% by mass) and the Al concentration (% by mass) in the sample after cooling were measured using an ICP emission spectrometer. The balance other than the Fe concentration and the Al concentration may be regarded as Zn.

The Fe concentration obtained by the ICP emission spectrometer is a so-called Total-Fe concentration including not only the Fe concentration (Free-Fe concentration) in the hot dip galvanizing bath but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the ICP emission spectrometer is a so-called Total-Al concentration including not only the Al concentration (Free-Al concentration) in the hot dip galvanizing bath but also the Al concentration in the dross. Therefore, the Free-Fe concentration and the Free-Al concentration are calculated using the obtained Total-Fe concentration and Total-Al concentration and a known Zn-Fe-Al ternary system state diagram. Specifically, a Zn-Fe-Al ternary system state diagram at the bath temperature at the time of sampling was prepared. As described above, the Zn-Fe-Al ternary system diagram is known, and it is also disclosed in FIG. 2 and FIG. 3 of non-patent document 1. Non-patent document 1 is a famous paper among researchers and developers of hot dip galvanizing baths. On the Zn-Fe-Al ternary system state diagram, specific points of Total-Fe concentration and Total-Al concentration obtained by ICP emission spectrum analysis were plotted. Then, connecting lines (conjugate lines) are drawn from the plotted points to the liquidus line in the Zn-Fe-Al ternary system state diagram. The Fe concentration at the intersection of the liquidus line and the connecting line corresponds to Free-Fe concentration, and the Al concentration at the intersection of the liquidus line and the connecting line corresponds to Free-Al concentration. By the above method, the Fe concentration (Free-Fe concentration) in the hot dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot dip galvanizing bath can be determined.

[ more preferred bath temperature for Hot-Dip galvanizing bath ]

The temperature (bath temperature) of the hot dip galvanizing bath 103 in the hot dip galvanizing treatment method is preferably 440 to 500 ℃. Dross in hot dip galvanizing bath 103 is mainly phase-changed into top dross (Fe) depending on the temperature of hot dip galvanizing bath 103 and the Al concentration in hot dip galvanizing bath 103₂Al₅Zn_x)、Γ₂Phase dross, delta₁Phase and zeta phase scum. Gamma-shaped₂Phase scum is easily generated in areas where the bath temperature is low. The zeta-phase scum is easy to be higher than gamma-phase scum at bath temperature₂Phase scum is generated in the area of the generation area.

Further, when the bath temperature of hot dip galvanizing bath 103 is 500 ℃ or lower, evaporation of Zn into mist can be suppressed. When the fumes are generated, the fumes adhere to the steel sheet and easily become surface flaws (fume flaws). The lower limit of the hot dip galvanizing bath 103 is preferably 460 ℃, more preferably 465 ℃, and still more preferably 469 ℃. The upper limit of the hot dip galvanizing bath 103 is preferably 490 ℃, more preferably 480 ℃, and still more preferably 475 ℃. Incidentally, the top dross tends to have an Al concentration higher than Γ₂The phase scum generation area and the zeta-phase scum generation area.

As described above, in the hot dip galvanizing treatment method of the present embodiment, a sample is collected from hot dip galvanizing bath 103 (sample collection step (S1)), and the amount of zeta-phase dross (zeta-phase) in hot dip galvanizing bath 103 is determinedA scum amount determination step (S2)). Then, the operating conditions of the hot dip galvanizing treatment are adjusted based on the amount of ζ -phase dross in the hot dip galvanizing bath 103 (operating condition adjusting step (S3)). By management with Γ₂The amount of the zeta-phase scum has a negative correlation with the amount of the phase scum, so that the operating conditions can be adjusted so as to suppress the generation of scum defects.

[ method for producing alloyed Hot-Dip galvanized Steel sheet ]

The hot-dip galvanizing treatment method according to the present embodiment described above can be applied to a method for manufacturing a galvannealed steel sheet (GA).

The method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment includes: a hot dip galvanizing treatment step and an alloying treatment step. In the hot-dip galvanizing process, the steel sheet is subjected to the above-described hot-dip galvanizing treatment method to form a hot-dip galvanized layer on the surface of the steel sheet. On the other hand, in the alloying step, the steel sheet having a hot-dip galvanized layer formed on the surface thereof in the hot-dip galvanizing step is subjected to alloying treatment using an alloying furnace 111 shown in fig. 2. The alloying treatment method may be any known method.

Through the above manufacturing steps, an alloyed hot-dip galvanized steel sheet can be manufactured. In the galvannealed steel sheet of the present embodiment, the hot-dip galvanizing treatment method of the present embodiment is used. That is, the operating conditions of the hot-dip galvanizing treatment are adjusted based on the ζ -phase dross amount, and the ζ -phase dross amount is increased. Thus, Γ in hot dip galvanizing bath 103₂The phase dross is relatively reduced, and as a result, the generation of dross defects in the produced alloyed hot-dip galvanized steel sheet can be suppressed.

The method of manufacturing the galvannealed steel sheet according to the present embodiment may include a manufacturing process other than the hot-dip galvanizing process and the alloying process. For example, the method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment may include, after the alloying treatment step, the following temper rolling step: temper rolling was performed using a temper rolling mill 30 shown in fig. 1. In this case, the appearance quality of the surface of the alloyed hot-dip galvanized steel sheet can be further improved. In addition, a manufacturing process other than the temper rolling process may be included.

[ method for producing Hot-Dip galvanized Steel sheet ]

The hot-dip galvanizing treatment method according to the present embodiment described above can be applied to a method for manufacturing a hot-dip galvanized steel sheet (GI).

The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes a hot-dip galvanizing treatment step. In the hot-dip galvanizing process, the steel sheet is subjected to the above-described hot-dip galvanizing treatment method to form a hot-dip galvanized layer on the surface of the steel sheet. In the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment, the hot-dip galvanizing treatment method according to the present embodiment is used. That is, the operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of ζ -phase dross, and the ζ -phase dross is increased. Therefore, the generation of dross defects in the produced hot-dip galvanized steel sheet can be suppressed.

The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment may include a manufacturing process other than the hot-dip galvanizing process. For example, the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment may include, after the hot-dip galvanizing treatment step, the following temper rolling step: temper rolling was performed using a temper rolling mill 30 shown in fig. 1. In this case, the appearance quality of the surface of the hot-dip galvanized steel sheet can be further improved. In addition, a manufacturing process other than the temper rolling process may be included.

Examples

Hereinafter, the effect of one mode of the hot-dip galvanizing treatment method according to the present embodiment will be described in more detail with reference to examples. The conditions in the examples are conditions employed for confirming the possibility of implementation and effects of the present invention. Therefore, the hot dip galvanizing treatment method according to the present embodiment is not limited to this conditional example.

In the above-described operation condition adjustment step, the relationship between the Fe concentration X and the Al concentration Y was examined.

Specifically, a hot-dip galvanizing treatment method is performed by a hot-dip galvanizing facility having the same configuration as that of fig. 2. Specifically, as shown in table 1, the Fe concentration X (mass%) and the Al concentration Y (mass%) of the hot dip galvanizing bath were adjusted. As the steel sheet, C: 0.003%, Si: 0.006%, Mn: 0.6%, P: 0.02%, S: 0.01% and the balance Fe and impurities. This high tensile steel is a so-called hard-to-alloy material which is difficult to alloy when producing an alloyed hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet is subjected to alloying treatment using an alloying furnace to produce an alloyed hot-dip galvanized steel sheet. The heating temperature in the alloying treatment was set constant (510 ℃) for any test number.

For each test number, in the hot dip galvanizing bath 103 of fig. 2, a sample was taken within a specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D. More specifically, in the hot dip galvanizing bath 103 of fig. 2, a sample is collected from a specific region (hereinafter, referred to as a sample collection region) defined by a specific depth range D107 in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L. In all of the test numbers, about 400g of the sample was collected from the same sample collection area. And cooling the collected sample to the normal temperature. The chemical composition of the hot dip galvanizing bath of each test number was measured by an ICP emission spectrometer using the cooled sample. The Fe concentration (% by mass) and the Al concentration (% by mass) obtained by the measurement were Total-Fe concentration (% by mass) and Total-Al concentration (% by mass). Therefore, the Total-Fe concentration and Total-Al concentration obtained and a known Zn-Fe-Al ternary system state diagram are used to calculate the Fe concentration (Free-Fe concentration) in the hot dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot dip galvanizing bath. Specifically, a Zn-Fe-Al ternary system state diagram at the bath temperature at the time of sampling was prepared. Specific points for Total-Fe concentration and Total-Al concentration obtained by ICP emission spectrometry are plotted on a known Zn-Fe-Al ternary system state diagram. A connecting line (conjugate line) is drawn from the plotted point to the liquidus line in the Zn-Fe-Al ternary system state diagram, and the intersection of the liquidus line and the connecting line is determined. The Fe concentration at the intersection was defined as Free-Fe concentration (% by mass), and the Al concentration at the intersection was defined as Free-Al concentration (% by mass). The Fe concentration (Free-Fe concentration) in the hot dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot dip galvanizing bath were determined by the above-described methods. As a result, the Fe concentration in the hot dip galvanizing bath was within a range of 0.02 to 0.05 mass% in any test number.

[ Table 1]

TABLE 1

In each test number, Al was appropriately added and adjusted over time so that the Fe concentration X (mass%) of the hot dip galvanizing bath became constant at the value shown in table 1 and the Al concentration Y (mass%) of the hot dip galvanizing bath became the concentration shown in table 1. The steel sheet conveyance speed in the hot dip galvanizing treatment was constant for any test number.

Table 1 also shows values on the right side of expressions (2) and (3). Further, it is described whether or not the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot dip galvanizing bath satisfy the formulas (1) to (3). For example, when a white circle (° c) is indicated in the column of formula (2), it indicates that the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot-dip galvanizing bath satisfy formula (2). When an index (X) is indicated in the column of formula (2), it indicates that the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot dip galvanizing bath do not satisfy formula (2).

In each test number, a sample was taken from the hot dip galvanizing bath under the operating conditions shown in table 1. Specifically, about 400g of the sample is collected from the sample collection area. From the collected sample, a test piece for observing ζ -phase scum was prepared. The test surface of the test piece for observing ζ -phase scum was set to 1cm × 1cm, and the thickness was set to 0.5 mm. The dross (top and bottom dross) was identified by contrast by observing the entire field of view of the test surface (1cm × 1cm) using a 100-fold SEM. Further, composition analysis using EPMA was carried out to classify the bottom sediment into Γ₂Phase dross, delta₁Phase scum and zeta-phase scum. Further, each of the specified bottom slags (Γ) is obtained₂Phase dross, delta₁Phase dross and zeta phase dross). Determining the number of pieces of zeta-phase scum with a circle equivalent diameter of 10 μm or more in the zeta-phase scum in the field of view of 1cm × 1cmAnd (4) counting. The number of zeta-phase scum with a circle equivalent diameter of 10 μm or more in the field of view (number/1 cm)²) As the amount of ζ -phase scum. The amount of the obtained ζ -phase scum is shown in table 1. In the present example, Γ was not observed in any test number₁And (4) phase scum.

[ evaluation test of dross Defect ]

After the hot dip galvanizing treatment was performed under the operating conditions of each test number, the alloying treatment was performed under the same conditions in each test number, and an alloyed hot dip galvanized steel sheet was manufactured. The surface of the produced alloyed hot-dip galvanized steel sheet was visually observed to examine the presence or absence of dross defects, and dross defects were evaluated. The criteria for evaluating the scum defect are as follows.

A: no scum defect (the number of the scum defect is 0/m)²)

B: the number of the scum defects exceeds 0 and is 0.1/m²The following

C: the number of scum defects exceeds 0.1/m²And is 1/m²The following

[ alloying evaluation test of difficult-to-alloy Material ]

The chemical composition of the alloyed hot-dip galvanized layer on the surface of the alloyed hot-dip galvanized steel sheet produced under the operating conditions of each test number was examined to evaluate the alloying of the material difficult to be alloyed. Specifically, the chemical composition of the alloyed hot-dip galvanized layer was analyzed by using an energy dispersive fluorescent X-ray analyzer (EDX-7000) manufactured by Shimadzu corporation. The alloying was evaluated by calculating the value obtained by dividing the Fe content (mass%) in the alloyed hot-dip galvanized layer by the Zn content (mass%) in the alloyed hot-dip galvanized layer. The criteria for the alloying evaluation are as follows. When the ratio of the Fe content to the Zn content is 11% or more, it is judged as an over-alloy.

A: the proportion of Fe content to Zn content is more than 10% and less than 11%

B: the proportion of Fe content to Zn content is more than 9% and less than 10%

C: the proportion of Fe content to Zn content is less than 9%

[ evaluation results ]

Referring to Table 1, the amount of ζ -phase dross was controlled to be 5.0 pieces/cm²In the above test numbers 1, 2, 5, 6, 8 to 13, the dross defect evaluation was a or B, and the dross defect was suppressed more effectively. In test nos. 1, 2, 5, 6, and 8 to 13, further, the alloying evaluation of the material difficult to alloy was a or B, and even if the material difficult to alloy was used, the alloying could be more effectively promoted. On the other hand, the amount of the zeta-phase scum is less than 5.0 pieces/cm²In test nos. 3, 4 and 7 of (1), the dross defect evaluation and the alloying evaluation of the difficult-to-alloy material were C. Further, referring to test nos. 1 to 13, the larger the amount of ζ -phase dross, the better the evaluation of dross defect. That is, the amount of ζ -phase dross and the number of dross defects show a negative correlation.

From the above results, it is understood that the scum defect can be suppressed by adjusting the operation conditions based on the amount of the ζ phase scum. It is also found that it is preferable that the threshold value of the amount of zeta-phase scum is 5.0 pieces/cm²And the amount of dross in the zeta phase is 5.0 pieces/cm²The operating conditions in the hot-dip galvanizing process are adjusted in the above manner, so that the dross defect can be significantly suppressed.

In test numbers 1, 2, 5, 6, 8 to 13 satisfying the formulas (1) and (2), the dross defect is evaluated as a or B, and the dross defect can be more effectively suppressed. In test nos. 1, 2, 5, 6, and 8 to 13, further, the alloying evaluation of the material difficult to alloy was a or B, and even if the material difficult to alloy was used, the alloying could be more effectively promoted. Therefore, it is found that adjustment so as to satisfy the formulas (1) and (2) is effective for suppressing dross defects and promoting alloying of a material difficult to alloy in adjustment of the operating conditions.

In test numbers 1, 5, 8, 9, 11, and 12 satisfying formula (1) and formula (3), the dross defect evaluation is a, and the dross defect can be further effectively suppressed. In test nos. 1, 5, 8, 9, 11 and 12, further, the alloying evaluation of the material difficult to alloy was a, and even if the material difficult to alloy was obtained, the alloying could be further effectively promoted. Therefore, it is found that, in adjusting the operating conditions, adjusting so as to satisfy the formulas (1) and (3) is further effective in suppressing the dross defect and promoting the alloying of the material difficult to alloy.

In test nos. 14 and 16 in which the Al concentration Y in the hot dip galvanizing bath was 0.0990 mass%, the dross defect was evaluated as "a", and alloying was promoted even for a material that is difficult to alloy, but alloying occurred in the production of the alloyed hot dip galvanized steel sheet. Therefore, it was revealed that the Al concentration Y in the hot dip galvanizing bath more preferably satisfies the formula (1).

In test nos. 15 and 17 in which the Al concentration Y in the hot dip galvanizing bath was 0.1410 mass%, the alloying evaluation of the material difficult to alloy was "C". Therefore, it was revealed that the Al concentration Y in the hot dip galvanizing bath more preferably satisfies the formula (1).

The embodiments of the present invention have been described above. However, the above embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriately changing the above embodiments without departing from the scope of the invention.

Description of the reference numerals

10 hot-dip galvanizing plant

101 molten zinc pot

103 hot dip galvanizing bath

107 sink roll

109 gas wiping device

111 alloying furnace

202 long nozzle

Claims

1. A hot-dip galvanizing treatment method, which is used to manufacture a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, and the hot-dip galvanizing treatment method has the following operations:

Sample collection process, collecting samples from a hot-dip galvanizing bath containing Al;

The step of determining the amount of ζ-phase dross, using the collected sample, to obtain the amount of ζ-phase dross in the hot-dip galvanizing bath; and,

In the operation condition adjustment step, the operation conditions of the hot-dip galvanizing treatment are adjusted based on the obtained amount of the zeta-phase dross.

2. The hot-dip galvanizing treatment method according to claim 1, wherein,

In the described ζ phase scum amount determination process,

Using the collected sample, the number of ζ-phase scum per predetermined area was determined as the ζ-phase scum amount.

3. The hot-dip galvanizing treatment method according to claim 1 or claim 2, wherein,

In the operating condition adjustment step,

Based on the obtained amount of the ζ-phase scum, at least one of (A) or (B) is performed to increase the amount of the ζ-phase scum,

(A) adjusting the bath temperature of the hot-dip galvanizing bath,

(B) Adjusting the Al concentration of the hot-dip galvanizing bath.

4. The hot-dip galvanizing method according to any one of claims 1 to 3, wherein:

In the operating condition adjustment step,

When the calculated amount of the ζ-phase scum is lower than a threshold value, the operating conditions of the hot-dip galvanizing process are adjusted to increase the amount of the ζ-phase scum.

5. The hot dip galvanizing treatment method according to claim 4, wherein,

In the described ζ phase scum amount determination process,

Using the collected sample, the number of z-phase scum per predetermined area is obtained as the z-phase scum amount,

In the operating condition adjustment step,

When the calculated amount of the zeta-phase scum is less than 5.0 pieces/cm ² in terms of unit area (1 cm ² ), the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the amount of scum. Describe the amount of ζ phase scum.

6. The hot-dip galvanizing method according to any one of claims 1 to 5, wherein:

In the operating condition adjustment step,

When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass %) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass %), the equations (1) and ( 2) to adjust the Fe concentration and Al concentration in the hot-dip galvanizing bath,

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2).

7. The hot-dip galvanizing treatment method according to claim 6, wherein,

In the operating condition adjustment step,

When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass %) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass %), the equations (1) and ( 3) to adjust the Fe concentration and the Al concentration in the hot-dip galvanizing bath,

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3).

8. The hot-dip galvanizing method according to any one of claims 1 to 7, wherein:

In the molten zinc pot in which the hot-dip galvanizing bath is stored, there are arranged submerged rolls for contacting the steel sheet dipped in the hot-dip galvanizing bath and switching the traveling direction of the steel sheet up and down ,

In the sample collection process,

The samples were taken from the depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the molten zinc pot.

9. A manufacturing method of alloyed hot-dip galvanized steel sheet, comprising the following steps:

a hot-dip galvanizing treatment process, wherein the hot-dip galvanizing treatment method according to any one of claims 1 to 8 is performed on a steel sheet, and a hot-dip galvanizing layer is formed on the surface of the steel sheet; and,

In the alloying treatment step, an alloying treatment is performed on the steel sheet having the hot-dip galvanized layer formed on the surface to manufacture the alloyed hot-dip galvanized steel sheet.

10 . A method for producing a hot-dip galvanized steel sheet, comprising a hot-dip galvanizing treatment step of applying the hot-dip galvanizing treatment method according to any one of claims 1 to 8 to a steel sheet, wherein the A hot-dip galvanized layer is formed on the surface of the steel sheet.